As is generally known, elongated profiles, such as sheet piles, tubes or the like, are driven into or pulled out of the ground with the aid of vibration. To this end use is made of vibratory actuators, also called vibration rams or vibrating units. Such vibration rams or vibrating units may be fastened to a sheet pile or any other body to be driven in a nonpositive manner by a grab. Once the pile is vibrated, it can be driven into the ground or can be pulled out of it with little application of force, or even by its own weight. In many embodiments known from the prior art two eccentric masses are mounted on at least two separate shafts. The shafts are connected to each other via gears such that the counterweights rotate in opposite directions in pairs.
Jolting that could damage adjacent structure should be avoided when vibration-driving or extracting piles. The risk of damage is particularly high in the region of the resonance frequency of the ground.
In order to avoid damage the prior art provides vibrating units operating at a significantly higher frequency than the resonance frequency of the ground, so-called high-frequency vibration rams. It has been shown that the propagation of vibrations in the ground caused by the pile is reduced as frequency of the vibration ram increases.
Furthermore, starting and stopping the vibrating unit creates low-value resonance frequencies. The prior art has therefore often proposed that vibratory actuators be set up such that the vibration amplitude of the device is significantly reduced or equal to zero while passing through certain resonant frequencies.
For this purpose one embodiment according to the prior art provides a pair of counterbalance shafts whose off-center masses may be adjusted on a further shaft relative to each other via a controller, such as an actuating motor. In one phase position of the unbalanced masses relative to each other the centrifugal forces cancel each other. In another phase position that is offset by 180° the centrifugal forces of the unbalanced masses are additive in a plane.
The amplitude of the vibrating unit is determined by the so-called static torque, divided by the weight of the vibrating mass. The static torque is calculated from the masses of the rotating unbalanced masses, multiplied by the spacing of the center of gravity of the individual counterbalanced masses from the rotation axis. The results show that a high static torque and/or a small vibrating total mass are required for a high amplitude. As the characteristic variable of the vibration ram the amplitude is dependent upon the rotational speed, or the vibration frequency of the vibration ram.
High-frequency vibrating units require physically large bearings in order to be able to support the high centrifugal forces per shaft, thus increasing the dynamic weight of the vibrating mass and therefore reducing the amplitude. The adjusting devices also increase the dynamic weight of the vibration rams such that an additional reduction of the amplitude takes place. Increasing the static torque for larger and heavier driving equipment is technically possible only by increasing the number of counterbalance shafts, which is associated with a proportional increase of the physical size of the vibrating unit. Due to the associated weight increase there is no improvement in amplitude.
A particular embodiment of vibratory actuators corresponding to the prior art is the so-called vertical vibrating unit. It is an embodiment having a narrow width so that a third pile can be set between two piles that are not yet driven in. The narrow width is achieved in that the individual counterbalance shafts are placed vertically on top of each other. According to the prior art four shafts are on top of each other for this purpose is order to eliminate any torques caused by the horizontal force. In this specific configuration a reduction of amplitude up to a zero value is achieved in that two unbalanced masses can be rotated relative to each other on one shaft. The preferred arrangement of the adjusting mechanism according to the prior art is an adjuster on an axis between two pairs of shafts. Due to this arrangement a relatively tall structure in the vibrating unit is the result. Since these types of vibrating units are usually mounted on an extension in a displaceable manner via a carriage, the maximum length of the piles to be placed is reduced by their construction height. An arrangement of unbalanced masses and shafts is known from patent application DE 196 31 991, which reduces the number of shafts required to three in order to create a low construction height.
U.S. Pat. No. 2,831,353 provides, for example, a high-power vibrating unit having a housing with two spaced bores, with two is counterweights rolling on the walls of the bores. The counterweights are each mounted on an arm, and the angle formed by the arms and therefore between the counterweights can be adjusted by a mechanism such that different counterbalanced torques may be set. For this purpose, however, neither the complicated adjusting mechanism, nor the simple guidance of the rollers in groves are suitable to satisfy the object described above of realizing a compact vibrating unit of high power at a high vibrating frequency. The object of the present invention is to create a device of the above stated type, which achieves a significantly higher amplitude of high-frequency vibration rams at a decreased physical size than is possible according to prior art, and which enables an adjustability of the vibration amplitude in an expanded embodiment.
A device is also known from DE 41 39 798 in which two shafts extending at a spacing and parallel to each other are provided in the housing thereof. An individually adjustable hydro-motor driving the shafts is associated with each of the shafts. Each shaft carries a counterweight in the form of a piston that is displaceable longitudinally in each of the hollow bodies parallel to the longitudinal axis in a relatively free-moving manner. The hollow body is integral with the respective shaft. The longitudinal axes of the different hollow bodies on different shafts are parallel to each other and orthogonal to a center line on which the center points of the shafts are positioned at a spacing from each other. Counterweights are longitudinally adjustable in the hollow bodies. In order to prevent the momentary generation of resonance frequencies at a high amplitude of the is vibratory actuator, the shafts are accelerated in the idle mode up to the desired rotational speed with the counterweights in neutral position. The counterweights are displaced into eccentric positions only after achieving the desired rotational speed.